They found that autistic children were three times more likely to have been exposed to high levels of traffic-related air pollution before their first birthday, and nearly twice as likely to have been exposed to those levels of pollution in the womb.

Pollution that caused the highest risk factor for autism was "particulate matter" — a mixture of acids, metals, soil and dust — and nitrogen dioxide, which is present in nearly all vehicle exhaust.

The researchers stressed that their study does not prove traffic pollution causes autism, only that it increases the risk of a child being diagnosed with the disorder.

"We're not saying traffic pollution causes autism, but it may be a risk factor for it," study leader Heather Volk said.

Volk said other environmental factors, including indoor pollution and passive smoking, could explain the results of her team's study.

Autism has baffled researchers for decades. A definitive cause has yet to be discovered but studies have revealed dozens of factors that increase risk, including heavy metal exposure, pesticides, obesity, flu or fever and prenatal stress, smoking, alcohol or illicit drug use.

Volk's study appears in the current issue of the Archives of General Psychiatry.